1. Field of the Invention
This invention relates to a ghost cancelling circuit and more particularly to the same for cancelling ghost which is multiple images on a TV screen arising from multipath signals.
2. Description of the Prior Art
When video signal is radiated from a transmitting station, the TV receiver receives the direct wave from there to the antenna and the reflected waves from building, mountain, airplane, etc. The reflected waves cover longer paths than the direct wave and hence are lower in signal level and delayed. Owing to generally receiving multipath signals including the many reflected, extra faint multiple images called ghost are produced in the horizontal vicinity of the desired image. Lowering of picture quality due to ghost is called ghost trouble.
In the prior art, ghost cancelling circuits for reducing or cancelling such ghost, as described, for example, in U.S. Pat. No. 4,953,026 (Ref. 1), comprises a ghost cancelling filter consisting of a variable or controllable delay circuit, and a transversal filter. Using the ghost cancelling filter a dummy ghost waveform is generated under the conditions where the delay time by the variable delay circuit and the tap gain by the transversal filter are so controlled that the dummy ghost waveform has the inverted characteristics to the ghost waveform, and then added to the input TV signal.
Besides in U.S. Pat. Nos. 4,897,725 (Ref. 2) and 4,947,252 (Ref. 3), Japanese Laid-Open Application No. Sho. 62-181579, and a paper "Development of a Ghost Cancel Technology for TV Broadcasting", NAB Engineering Conference Proceedings 1990, pp. 229-238 (Ref. 4) is described a ghost cancelling circuit for NTSC signals, and ghost cancel reference (GCR) signals are inserted in the received video signals during the vertical blanking time. In the circuit of Ref. 2 as typical example of these conventional ghost cancelling circuits. A Fourier transform of the difference between the GCR signal and the previously stored standard waveform of the GCR signal is obtained, and then inverse Fourier transform of the resulting Fourier transform is obtained. Thereby the delay time and tap gain of the ghost cancelling filter are set.
Ghost cancellation technique by insertion of GCR signals will be more detailed below. In broadcasting stations, GCR signals are inserted during the vertical blanking time of NTSC signals as described above. In the receiver, the GCR signals including ghost are extracted from the received signals, and the waveform distortion of them are analyzed to obtain the characteristics of the ghost. The analysis consists of Fourier transform of the difference between the received GCR signal and the reference waveform of the previously stored GCR signal and then the inverse Fourier transform of the resultant Fourier transform as described Refs. 3 and 4. The output of the inverse transform controls the delay and tap gain of the ghost cancelling filter, and under this conditions, the filter exhibits a pass characteristic of deleting the amplitude of the received ghost on the time axis. The received signals therefore are passed through the ghost cancelling filter to filter the ghost from them. The ghost cancelling filter is composed of a nonrecursive filter part for cancelling nearby-ghost and a recursive filter part for cancelling the other ghosts as described in Ref. 3. As well-known, these filter parts are a digital filter consisting of an adder, a coefficient multiplier, and a unit-delay component. Nonrecursive filter is also called transversal filter.
Those ghost cancelling circuits in the prior art is used for television signals of NTSC system or similar standards (referred to as standard signals hereinafter) regardless less of whether the GCR signals are inserted or not. TV signals from which ghost can be canceled by these cancelling circuits are of NTSC system or similar standards including subcarrier signal (frequency f.sub.sc =3579545.+-.10 Hz for NTSC system), and besides horizontal synchronizing signal (frequency f.sub.H =2.times.f.sub.sc /455=15.734 KHz for NTSC) and vertical synchronizing signal (frequency f.sub.v =2f.sub.H /525=59,94 Hz for NTSC) synchronous in phase with the subcarrier signal.
When non-standard signals other than the standard signals are input, the ghost cancelling operation can not be caused under the condition of having established the synchronism with the TV signals because the time base for waveform analysis is different from the standard signals. On the other hand, practically TV receivers on which non-standard signals as of video cassette recorder (VCR), laser disk (LD) player, and television game are much displayed, and therefore which is provided with a conventional-type ghost cancelling circuit can not reproduce these non-standard signals as normal TV pictures when input.